AIMS/HYPOTHESIS: We evaluated seasonal HbA(1c) changes in children with type 1 diabetes and its relation with measures of weather conditions. METHODS: HbA(1c) changes over more than 3 years were evaluated in type 1 diabetic patients who were younger than 18 years and had diabetes duration of more than 12 months, and correlated with measures of weather conditions (ambient temperature, hours of sunshine and solar irradiance). After comparison of autocorrelation patterns, patterns of metabolic control and meteorological data were evaluated using Spearman rank correlation. RESULTS: A total of 3,935 HbA(1c) measurements in 589 school (>/Euro Surveillance (Bulletin Europeen Sur Les Maladies Transmissibles; European Communicable Disease Bulletin) 7 years) and 88 preschool (<7 years) children were analysed. Mean (+/- SD) HbA(1c) level for the whole study period was 7.65 +/- 1.12%. The lowest HbA(1c) levels were observed in late summer and the highest in winter months, with differences consistently exceeding 0.44%. Autocorrelation analysis of HbA(1c) levels in schoolchildren showed a sine-wave pattern with a cycle length of roughly 12 months, which mirrored changes in ambient temperature. Strong negative correlations of HbA(1c) with ambient temperature (R Euro Surveillance (Bulletin Europeen Sur Les Maladies Transmissibles; European Communicable Disease Bulletin) -0.56; p Euro Surveillance (Bulletin Europeen Sur Les Maladies Transmissibles; European Communicable Disease Bulletin) 0.0002), hours of sunshine (REuro Surveillance (Bulletin Europeen Sur Les Maladies Transmissibles; European Communicable Disease Bulletin) -0.52; p Euro Surveillance (Bulletin Europeen Sur Les Maladies Transmissibles; European Communicable Disease Bulletin) 0.0007) and solar irradiance (R Euro Surveillance (Bulletin Europeen Sur Les Maladies Transmissibles; European Communicable Disease Bulletin) -0.52; p Euro Surveillance (Bulletin Europeen Sur Les Maladies Transmissibles; European Communicable Disease Bulletin) 0.0006) were present in schoolchildren, but not in preschoolers (p >/Euro Surveillance (Bulletin Europeen Sur Les Maladies Transmissibles; European Communicable Disease Bulletin) 0.29 for each correlation). CONCLUSIONS/INTERPRETATION: Seasonal changes of HbA(1c) levels in schoolchildren with type 1 diabetes are a significant phenomenon and should be considered in patient education and diabetes management. They may potentially affect the results of clinical trials using HbA(1c) levels as their primary outcome, as well as HbA(1c)-based diagnosis of diabetes.